Heart disease is the leading cause of death in the United States. A heart attack (also known as an Acute Myocardial Infarction (AMI)) typically results from a thrombus that obstructs blood flow in one or more coronary arteries. AMI is a common and life-threatening complication of coronary heart disease. The sooner that perfusion of the myocardium is restored (e.g., with injection of a thrombolytic medication such as tissue plasminogen activator (tPA)), the better the prognosis and survival of the patient from the heart attack. The extent of damage to the myocardium is strongly dependent upon the length of time prior to restoration of blood flow to the heart muscle.
Myocardial ischemia is caused by a temporary imbalance of blood (oxygen) supply and demand in the heart muscle. It is typically provoked by physical activity or other causes of increased heart rate when one or more of the coronary arteries are obstructed by atherosclerosis. Patients will often (but not always) experience chest discomfort (angina) when the heart muscle is experiencing ischemia.
Acute myocardial infarction and ischemia may be detected from a patient""s electrocardiogram (ECG) by noting an ST segment shift (i.e., voltage change) over a relatively short (less than 5 minutes) period of time. However, without knowing the patient""s normal ECG pattern detection from standard 12 lead ECG can be unreliable. In addition, ideal placement of subcutaneous electrodes for detection of ST segment shifts as they would relate to a subcutaneously implanted device has not been explored in the prior art.
Fischell et al in U.S. Pat. Nos. 6,112,116 and 6,272,379 describe implantable systems for detecting the onset of acute myocardial infarction and providing both treatment and alarming to the patient. While Fischell et al discuss the detection of a shift in the S-T segment of the patient""s electrogram from an electrode within the heart as the trigger for alarms; it may be desirable to provide more sophisticated detection algorithms to reduce the probability of false positive and false negative detection. In addition while these patents describe some desirable aspects of programming such systems, it may be desirable to provide additional programmability and alarm control features.
Although anti-tachycardia pacemakers and Implantable Cardiac Defibrillators (ICDs) can detect heart arrhythmias, none are currently designed to detect ischemia and acute myocardial infarction events independently or in conjunction with arrhythmias.
In U.S. Pat. Nos. 6,112,116 and 6,272,379 Fischell et al, discuss the storage of recorded electrogram and/or electrocardiogram data; however techniques to optimally store the appropriate electrogram and/or electrocardiogram data and other appropriate data in a limited amount of system memory are not detailed.
In U.S. Pat. No. 5,497,780 by M. Zehender, a device is described that has a xe2x80x9cgoal of eliminating . . . cardiac rhythm abnormality.xe2x80x9d To do this, Zehender requires exactly two electrodes placed within the heart and exactly one electrode placed outside the heart. Although multiple electrodes could be used, the most practical sensor for providing an electrogram to detect a heart attack would use a single electrode placed within or near to the heart.
Zehender""s drawing of the algorithm consists of a single box labeled ST SIGNAL ANALYSIS with no details of what the analysis comprises. His only description of his detection algorithm is to use a comparison of the ECG to a reference signal of a normal ECG curve. Zehender does not discuss any details to teach an algorithm by which such a comparison can be made, nor does Zehender explain how one identifies the xe2x80x9cnormal ECG curvexe2x80x9d. Each patient will likely have a different xe2x80x9cnormalxe2x80x9d baseline ECG that will be an essential part of any system or algorithm for detection of a heart attack or ischemia.
In addition, Zehender suggests that an ST signal analysis should be carried out every three minutes. It may be desirable to use both longer and shorter time intervals than 3 minutes so as to capture certain changes in ECG that are seen early on or later on in the evolution of an acute myocardial infarction. Longer observation periods will also be important to account for minor slowly evolving changes in the xe2x80x9cbaselinexe2x80x9d ECG. Zehender has no mention of detection of ischemia having different normal curves based on heart rate. To differentiate from exercise induced ischemia and acute myocardial infarction, it may be important to correlate ST segment shifts with heart rate or Rxe2x80x94R interval.
Finally, Zehender teaches that xe2x80x9cif an insufficient blood supply in comparison to the reference signal occurs, the corresponding abnormal ST segments can be stored in the memory in digital form or as a numerical event in order to be available for associated telemetry at any time.xe2x80x9d Storing only abnormal ECG segments may miss important changes in baseline ECG. Thus it is desirable to store some historical ECG segments in memory even if they are not xe2x80x9cabnormalxe2x80x9d.
The Reveal(trademark) subcutaneous loop Holter monitor sold by Medtronic uses two case electrodes spaced by about 3 inches to record electrocardiogram information looking for arrhythmias. It has no real capability to detect ST segment shift and its high pass filtering would in fact preclude accurate detection of changes in the low frequency aspects of the heart""s electrical signal. Also the spacing of the electrodes it too close together to be able to effectively detect and record ST segment shifts. Similarly, current external Holter monitors are primarily designed for capturing arrhythmia related signals from the heart.
Although often described as an electrocardiogram (ECG), the stored electrical signal from the heart as measured from electrodes within the body should be termed an xe2x80x9celectrogramxe2x80x9d. The early detection of an acute myocardial infarction or exercise induced myocardial ischemia caused by an increased heart rate or exertion is feasible using a system that notes a change in a patient""s electrogram. The portion of such a system that includes the means to detect a cardiac event is defined herein as a xe2x80x9ccardiosaverxe2x80x9d and the entire system including the cardiosaver and the external portions of the system is defined herein as a xe2x80x9cguardian system.xe2x80x9d
Furthermore, although the masculine pronouns xe2x80x9chexe2x80x9d and xe2x80x9chisxe2x80x9d are used herein, it should be understood that the patient or the medical practitioner who treats the patient could be a man or a woman. Still further the term; xe2x80x9cmedical practitionerxe2x80x9d shall be used herein to mean any person who might be involved in the medical treatment of a patient. Such a medical practitioner would include, but is not limited to, a medical doctor (e.g., a general practice physician, an internist or a cardiologist), a medical technician, a paramedic, a nurse or an electrogram analyst. A xe2x80x9ccardiac eventxe2x80x9d includes an acute myocardial infarction, ischemia caused by effort (such as exercise) and/or an elevated heart rate, bradycardia, tachycardia or an arrhythmia such as atrial fibrillation, atrial flutter, ventricular fibrillation, and premature ventricular or atrial contractions (PVCs or PACs).
For the purposes of this specification, the terms xe2x80x9cdetectionxe2x80x9d and xe2x80x9cidentificationxe2x80x9d of a cardiac event have the same meaning.
For the purpose of this invention, the term xe2x80x9celectrocardiogramxe2x80x9d is defined to be the heart electrical signals from one or more skin surface electrode(s) that are placed in a position to indicate the heart""s electrical activity (depolarization and repolarization). An electrocardiogram segment refers to the recording of electrocardiogram data for either a specific length of time, such as 10 seconds, or a specific number of heart beats, such as 10 beats. For the purposes of this specification the PQ segment of a patient""s electrocardiogram is the typically flat segment of a beat of an electrocardiogram that occurs just before the R wave.
For the purpose of this invention, the term xe2x80x9celectrogramxe2x80x9d is defined to be the heart electrical signals from one or more implanted electrode(s) that are placed in a position to indicate the heart""s electrical activity (depolarization and repolarization). An electrogram segment refers to the recording of electrogram data for either a specific length of time, such as 10 seconds, or a specific number of heart beats, such as 10 beats. For the purposes of this specification the PQ segment of a patient""s electrogram is the typically flat segment of an electrogram that occurs just before the R wave. For the purposes of this specification, the terms xe2x80x9cdetectionxe2x80x9d and xe2x80x9cidentificationxe2x80x9d of a cardiac event have the same meaning. A beat is defined as a sub-segment of an electrogram or electrocardiogram segment containing exactly one R wave.
Heart signal parameters are defined to be any measured or calculated value created during the processing of one or more beats of electrogram data. Heart signal parameters include PQ segment average value, ST segment average value, R wave peak value, ST deviation, ST shift, average signal strength, T wave peak height, T wave average value, T wave deviation, heart rate and Rxe2x80x94R interval.
The present invention is a system for the detection of cardiac events (a guardian system) that includes a device called a cardiosaver, a physician""s programmer and an external alarm system. The present invention envisions a system for early detection of an acute myocardial infarction or exercise induced myocardial ischemia caused by an increased heart rate or exertion. In the preferred embodiment of the present invention, the cardiosaver is implanted along with the electrodes. In an alternate embodiment, the cardiosaver and the electrodes could be external but attached to the patient""s body. Although the following descriptions of the present invention in most cases refer to the preferred embodiment of an implanted cardiosaver processing electrogram data from implanted electrodes, the techniques described are equally applicable to the alternate embodiment where the external cardiosaver processes electrocardiogram data from skin surface electrodes.
In the preferred embodiment of the cardiosaver either or both subcutaneous electrodes or electrodes located on a pacemaker type right ventricular or atrial leads will be used. It is also envisioned that one or more electrodes may be placed within the superior vena cava. One version of the implanted cardiosaver device using subcutaneous electrodes would have an electrode located under the skin on the patient""s left side. This could be best located between 2 and 20 inches below the patient""s left arm pit. The cardiosaver case that would act as the indifferent electrode would typically be implanted like a pacemaker under the skin on the left side of the patient""s chest.
Using one or more detection algorithms, the cardiosaver can detect a change in the patient""s electrogram that is indicative of a cardiac event, such as an acute myocardial infarction, within five minutes after it occurs and then automatically warn the patient that the event is occurring. To provide this warning, the guardian system includes an internal alarm sub-system (internal alarm means) within the cardiosaver and/or an external alarm system (external alarm means). In the preferred, implanted embodiment, the cardiosaver communicates with the external alarm system using a wireless radio-frequency (RF) signal.
The internal alarm means generates an internal alarm signal to warn the patient. The internal alarm signal may be a mechanical vibration, a sound or a subcutaneous electrical tickle. The external alarm system (external alarm means) will generate an external alarm signal to warn the patient. The external alarm signal is typically a sound that can be used alone or in combination with the internal alarm signal. The internal or external alarm signals would be used to alert the patient to at least two different types of conditions: a major event alarm signaling the detection of a major cardiac event (e.g. a heart attack) and the need for immediate medical attention, and a less critical xe2x80x9cSEE DOCTORxe2x80x9d alarm signaling the detection of a less serious non life threatening condition such as exercise induced ischemia. The SEE DOCTOR alarm signal would be used to tell the patient that he is not in immediate danger but should arrange an appointment with his doctor in the near future. In addition to the signaling of less critical cardiac events, the SEE DOCTOR alarm signal could also signal the patient when the cardiosaver battery is getting low.
In the preferred embodiment, in a major event alarm the internal alarm signal would be applied periodically, for example, with three pulses every 5 seconds after the detection of a major cardiac event. It is also envisioned that the less critical xe2x80x9cSEE DOCTORxe2x80x9d alarm, would be signaled in a different way, such as one pulse every 7 seconds.
The external alarm system is a hand-held portable device that may include any or all the following features:
1. an external alarm means to generate an external alarm signal to alert the patient.
2. the capability to receive cardiac event alarm, recorded electrogram and other data from the cardiosaver
3. the capability to transmit the cardiac event alarm, recorded electrogram and other data collected by the cardiosaver to a medical practitioner at a remote location.
4. an xe2x80x9calarm-offxe2x80x9d button that when depressed can acknowledge that the patient is aware of the alarm and will turn off internal and external alarm signals.
5. a display (typically an LCD panel) to provide information and/or instructions to the patient by a text message and the display of segments of the patient""s electrogram.
6. the ability to provide messages including instructions to the patient via a pre-recorded human voice.
7. a patient initiated electrogram capture initiated by a xe2x80x9cPanic Buttonxe2x80x9d to allow the patient, even when there has been no alarm, to initiate transmission of electrogram data from the cardiosaver to the external alarm system for transmission to a medical practitioner.
8. a patient initiated electrogram capture to initiate transmission of electrogram data from the cardiosaver to the external alarm system for display to a medical practitioner using the display on the external alarm system.
9. the capability to automatically turn the internal and external alarms off after a reasonable time period that is typically less than 30 minutes if the alarm-off button is not used.
Text and/or spoken instructions may include a message that the patient should promptly take some predetermined medication such as chewing an aspirin, placing a nitroglycerine tablet under his tongue, inhaling or nasal spraying a single or multiple drug combination and/or injecting thrombolytic drugs into a subcutaneous drug port. The messaging displayed by or spoken from the external alarm system and/or a phone call from a medical practitioner who receives the alarm could also inform the patient that he should wait for the arrival of emergency medical services or he should promptly proceed to an emergency medical facility. It is envisioned that the external alarm system can have direct connection to a telephone line and/or work through cell phone or other wireless networks.
If a patient seeks care in an emergency room, the external alarm system could provide a display to the medical practitioners in the emergency room of both the electrogram segment that caused the alarm and the baseline electrogram segment against which the electrogram that caused the alarm was compared. The ability to display both baseline and alarm electrogram segments will significantly improve the ability of the emergency room physician to properly identify AMI.
The preferred embodiment of the external alarm system consists of an external alarm transceiver and a handheld computer. The external alarm transceiver having a standardized interface, such as Compact Flash adapter interface, a secure digital (SD) card interface, a multi-media card interface, a memory stick interface or a PCMCIA card interface. The standardized interface will allow the external alarm transceiver to connect into a similar standardized interface slot that is present in many handheld computers such as a Palm Pilot or Pocket PC. An advantage of this embodiment is that the handheld computer can cost effectively supply the capability for text and graphics display and for playing spoken messages.
Using a handheld computer, such as the Thera(trademark) by Audiovox(trademark) that combines a Pocket PC with having an SD/Multimedia interface slot with a cell phone having wireless internet access, is a solution that can easily be programmed to provide communication between the external alarm system and a diagnostic center staffed with medical practitioners.
The panic button feature, which allows a patient-initiated electrogram capture and transmission to a medical practitioner, will provide the patient with a sense of security knowing that, if he detects symptoms of a heart-related ailment such as left arm pain, chest pain or palpitations, he can get a fast review of his electrogram. Such a review would allow the diagnosis of arrhythmias, such as premature atrial or ventricular beats, atrial fibrillation, atrial flutter or other heart rhythm irregularities. The medical practitioner could then advise the patient what action, if any, should be taken. The guardian system would also be programmed to send an alarm in the case of ventricular fibrillation so that a caretaker of the patient could be informed to immediately provide a defibrillation electrical stimulus. This is practical as home defibrillation units are now commercially available. It is also possible that, in patients prone to ventricular fibrillation following a myocardial infarction, such a home defibrillator could be placed on the patient""s chest to allow rapid defibrillation should ventricular fibrillation occur while waiting for the emergency medical services to arrive.
The physician""s programmer provides the patient""s doctor with the capability to set cardiosaver cardiac event detection parameters. The programmer communicates with the cardiosaver using the wireless communication capability that also allows the external alarm system to communicate with the cardiosaver. The programmer can also be used to upload and review electrogram data captured by the cardiosaver including electrogram segments captured before, during and after a cardiac event.
An extremely important capability of the present invention is the use of a continuously adapting cardiac event detection program that compares extracted features from a recently captured electrogram segment with the same features extracted from a baseline electrogram segment at a predetermined time in the past. For example, the thresholds for detecting an excessive ST shift would be appropriately adjusted to account for slow changes in electrode sensitivity or ST segment levels over time. It may also be desirable to choose the predetermined time in the past for comparison to take into account daily cycles in the patient""s heart electrical signals. Thus, a preferred embodiment of the present invention would use a baseline for comparison that is collected approximately 24 hours prior to the electrogram segment being examined. Such a system would adapt to both minor (benign) slow changes in the patient""s baseline electrogram as well as any daily cycle.
Use of a system that adapts to slowly changing baseline conditions is of great importance in the time following the implantation of electrode leads in the heart. This is because there can be a significant xe2x80x9cinjury currentxe2x80x9d present just after implantation of an electrode and for a time of up to a month, as the implanted electrode heals into the wall of the heart. Such an injury current may produce a depressed ST segment that deviates from a normal isoelectric electrogram where the PQ and ST segments are at approximately the same voltage. Although the ST segment may be depressed due to this injury current, the occurrence of an acute myocardial infarction can still be detected since an acute myocardial infarction will still cause a significant shift from this xe2x80x9cinjury currentxe2x80x9d ST baseline electrogram. Alternately, the present invention might be implanted and the detector could be turned on after healing of the electrodes into the wall of the heart. This healing would be noted in most cases by the evolution to an isoelectric electrogram (i.e., PQ and ST segments with approximately the same voltages).
The present invention""s ST detection technique involves recording and processing baseline electrogram segments to calculate the threshold for myocardial infarction and/or ischemia detection. These baseline electrogram segments would typically be collected, processed and stored once an hour or with any other appropriate time interval.
A preferred embodiment of the present invention would save and process a 10 second baseline electrogram segment once every hour. Every 30 seconds the cardiosaver would save and process a 10 second long recent electrogram segment. The cardiosaver would compare the recent electrogram segment with the baseline electrogram segment from approximately 24 hours before (i.e. 24xc2x1xc2xd hour before).
The processing of each of the hourly baseline electrogram segments would involve calculating the average electrogram signal strength as well as calculating the average xe2x80x9cST deviationxe2x80x9d. The ST deviation for a single beat of an electrogram segment is defined to be the difference between the average ST segment voltage and the average PQ segment voltage. The average ST deviation of the baseline electrogram segment is the average of the ST deviation of multiple (at least two) beats within the baseline electrogram segment.
The following detailed description of the drawings fully describes how the ST and PQ segments are measured and averaged.
An important aspect of the present invention is the capability to adjust the location in time and duration of the ST and PQ segments used for the calculation of ST shifts. The present invention is initially programmed with the time interval between peak of the R wave of a beat and the start of the PQ and ST segments of that beat set for the patient""s normal heart rate. As the patient""s heart rate changes during daily activities, the present invention will adjust these time intervals for each beat proportional to the Rxe2x80x94R interval for that beat. In other words, if the Rxe2x80x94R interval shortens (higher heart rate) then the ST and PQ segments would move closer to the R wave peak and would become shorter. ST and PQ segments of a beat within an electrogram segment are defined herein as sub-segments of the electrogram segment.
The difference between the ST deviation on any single beat in a recently collected electrogram segment and a baseline average ST deviation extracted from a baseline electrogram segment is defined herein as the xe2x80x9cST shiftxe2x80x9d for that beat. The present invention envisions that detection of acute myocardial infarction and/or ischemia would be based on comparing the ST shift of one or more beats with a predetermined detection threshold xe2x80x9cHSTxe2x80x9d.
In U.S. application Ser. No. 1,005,1743 that is incorporated herein by reference, Fischell describes a fixed threshold for detection that is programmed by the patient""s doctor. The present invention envisions that the threshold should rather be based on some percentage xe2x80x9cPSTxe2x80x9d of the average signal strength extracted from the baseline electrogram segment where PST is a programmable parameter of the cardiosaver device. The xe2x80x9csignal strengthxe2x80x9d can be measured as peak signal voltage, RMS signal voltage or as some other indication of signal strength such as the difference between the average PQ segment amplitude and the peak R wave amplitude.
Similarly, it is envisioned that the value of PST might be adjusted as a function of heart rate so that a higher threshold could be used if the heart rate is elevated, so as to not trigger on exercise that in some patients will cause minor ST segment shifts when there is not a heart attack occurring. Alternately, lower thresholds might be used with higher heart rates to enhance sensitivity to detect exercise-induced ischemia. One embodiment of the present invention has a table stored in memory where values of PST for a preset number of heart rate ranges, (e.g. 50-80, 81-90, 91-100, 101-120, 121-140) might be stored for use by the cardiosaver detection algorithm in determining if an acute myocardial infarction or exercise induced ischemia is present.
Thus it is envisioned that the present invention would use the baseline electrogram segments in 3 ways.
1. To calculate a baseline average value of a feature such as ST deviation that is then subtracted from the value of the same feature in recently captured electrogram segments to calculate the shift in the value of that feature. E.g. the baseline average ST deviation is subtracted from the amplitude of the ST deviation on each beat in a recently captured electrogram segment to yield the ST shift for that beat.
2. To provide an average signal strength used in calculating the threshold for detection of a cardiac event. This will improve detection by compensating for slow changes in electrogram signal strength over relatively long periods of time.
3. To provide a medical practitioner with information that will facilitate diagnosis of the patient""s condition. For example, the baseline electrogram segment may be transmitted to a remotely located medical practitioner and/or displayed directly to a medical practitioner in the emergency room.
For the purposes of the present invention, the term adaptive detection algorithm is hereby defined as a detection algorithm for a cardiac event where at least one detection-related threshold adapts over time so as to compensate for relatively slow (longer than an hour) changes in the patient""s normal electrogram.
It is also envisioned that the present invention could have specific programming to identify a very low heart rate (bradycardia) or a very high heart rate (tachycardia or fibrillation). While a very low heart rate is usually not of immediate danger to the patient, its persistence could indicate the need for a pacemaker. As a result, the present invention could use the xe2x80x9cSEE DOCTORxe2x80x9d alarm along with an optional message sent to the external alarm system to alert the patient that his heart rate is too low and that he should see his doctor as soon as convenient. On the other hand, a very high heart rate can signal immediate danger thus it would be desirable to alarm the patient in a manner similar to that of acute myocardial infarction detection. What is more, detections of excessive ST shift during high heart rates may be difficult and if the high heart rate is the result of a heart attack then it is envisioned that the programming of the present invention would use a major event counter that would turn on the alarm if the device detects a combination of excessive ST shift and overly high heart rate.
Another early indication of acute myocardial infarction is a rapid change in the morphology of the T wave. Unfortunately, there are many non-AMI causes of changes in the morphology of a T wave. However, these changes typically occur slowly while the changes from an AMI occur rapidly. Therefore one embodiment of this invention uses detection of a change in the T wave as compared to a baseline collected a short time (less than 30 minutes) in the past. The best embodiment is probably using a baseline collected between 1 and 5 minutes in the past. Such a T wave detector could look at the amplitude of the peak of the T wave. An alternate embodiment of the T wave detector might look at the average value of the entire T wave as compared to the baseline. The threshold for T wave shift detection, like that of ST shift detection, can be a percentage PT of the average signal strength of the baseline electrogram segment. PT could differ from PST if both detectors are used simultaneously by the cardiosaver.
In its simplest form, the xe2x80x9cguardian systemxe2x80x9d includes only the cardiosaver and a physician""s programmer. Although the cardiosaver could function without an external alarm system where the internal alarm signal stays on for a preset period of time, the external alarm system is highly desirable. One reason it is desirable is the button on the external alarm system that provides the means for of turning off the alarm in either or both the implanted device (cardiosaver) and the external alarm system. Another very important function of the external alarm system is to facilitate display of both the baseline and alarm electrogram segments to a treating physician to facilitate rapid diagnosis and treatment for the patient.
Thus it is an object of this invention is to have a cardiosaver designed to detect the occurrence of a cardiac event by comparing baseline electrogram data from a first predetermined time with recent electrogram data from a second predetermined time.
Another object of the present invention is to have a cardiac event detected by comparing at least one heart signal parameter extracted from an electrogram segment captured at a first predetermined time by an implantable cardiosaver with the same at least one heart signal parameter extracted from an electrogram segment captured at a second predetermined time.
Another object of the present invention is to have acute myocardial infarction detected by comparing recent electrogram data to baseline electrogram data from the same time of day (i.e. approximately 24 hours in the past).
Another object of the present invention is to have acute myocardial infarction detected by comparing the ST deviation of the beats in a recently collected electrogram segment to the average ST deviation of two or more beats of a baseline electrogram segment.
Another object of the present invention is to have the threshold(s) for detecting the occurrence of a cardiac event adjusted by a cardiosaver device to compensate for slow changes in the average signal level of the patient""s electrogram.
Another object of the present invention is to have the threshold for detection of a cardiac event adjusted by a cardiosaver device to compensate for daily cyclic changes in the average signal level of the patient""s electrogram.
Another object of the present invention is to have an external alarm system including an alarm off button that will turn off either or both internal and external alarm signals initiated by an implanted cardiosaver.
Another object of the present invention is to have the alarm signal generated by a cardiosaver automatically turn off after a preset period of time.
Still another object of this invention is to use the cardiosaver to warn the patient that an acute myocardial infarction has occurred by means of a subcutaneous vibration.
Still another object of this invention is to have the cardiac event detection require that at least a majority of the beats exhibit an excessive ST shift before identifying an acute myocardial infarction.
Still another object of this invention is to have the cardiac event detection require that excessive ST shift still be present in at least two electrogram segments separated by a preset period of time.
Still another object of this invention is to have the cardiac event detection require that excessive ST shift still be present in at least three electrogram segments separated by preset periods of time.
Yet another object of the present invention is to have a threshold for detection of excessive ST shift that is dependent upon the average signal strength calculated from a baseline electrogram segment.
Yet another object of the present invention is to have a threshold for detection of excessive ST shift that is a function of the difference between the average PQ segment amplitude and the R wave peak amplitude of a baseline electrogram segment.
Yet another object of the present invention is to have a threshold for detection of excessive ST shift that is a function of the average minimum to maximum amplitude for at least two beats calculated from a baseline electrogram segment.
Yet another object of the present invention is to have the ability to detect a cardiac event by the shift in the amplitude of the T wave of an electrogram segment at a second predetermined time as compared with the average baseline T wave amplitude from a baseline electrogram segment at a first predetermined time.
Yet another object of the present invention is to have the ability to detect a cardiac event by the shift in the T wave deviation of at least one beat of an electrogram segment at a second predetermined time as compared with the average baseline T wave deviation from an electrogram segment at a first predetermined time.
Yet another object of the present invention is to have the first and second predetermined times for T wave amplitude and/or deviation comparison be separated by less than 30 minutes.
Yet another object of the present invention is to have the baseline electrogram segment used for ST segment shift detection and the baseline electrogram segment used for T wave shift detection be collected at different times.
Yet another object of the present invention is to have an individualized (patient specific) xe2x80x9cnormalxe2x80x9d heart rate range such that the upper and lower limits of xe2x80x9cnormalxe2x80x9d are programmable using the cardiosaver programmer.
Yet another object of the present invention is to have one or more individualized (patient specific) xe2x80x9celevatedxe2x80x9d heart rate ranges such that the upper and lower limits of each xe2x80x9celevatedxe2x80x9d range are programmable using the cardiosaver programmer.
Yet another object of the present invention is to allow the threshold for detection of an excessive ST shift be different for the xe2x80x9cnormalxe2x80x9d heart rate range as compared to one or more xe2x80x9celevatedxe2x80x9d heart rate ranges.